ABSTRACT ? COMPARATIVE PATHOLOGY CORE Vertebrate animal model use is essential to answer fundamental questions and elucidate the mechanisms in vivo through which environmental exposures may influence human disease and produce adverse human health outcomes. Diverse animal models, such as zebrafish, medaka fish, mouse, rat, rabbit, guinea pig, vole, xenopus, domestic animal (dogs, pigs, sheep, etc.) and others, along with their genetically engineered counterparts, represent powerful in vivo approaches to link molecular mechanisms to pathologic lesions (disrupted organ/tissue function) and toxic effects. Animal models remain the gold standard for studying these effects of environmental toxicants on development, cancer, reproduction, organ toxicity, and neurological and endocrine systems. These approaches are powerful because in vivo model organisms operate in a systems biology framework integrating all levels of biological organization - biomolecule, pathway, cell, tissue, organ, and organism - and inform us of potential adverse human health outcomes. Understanding these adverse effects requires expert pathologic evaluation of the affected tissue/organ/cells. To fill this need, the mission of the Comparative Pathology Core (CPC) is to provide CHHE member scientists dedicated access to cost-free pathology expertise to assess the effects of environmental factors in a diverse range of animal models.